44 ALUMINIUM SULPHATE quotient obtained by dividing the percentage of sulphuric acid (H2S04) by that of aluminium (Al). 4. Free Sulphuric Acid.—According to Iwanow,1 25 c.c. of the solu- tion (i gram of substance) are heated to about 85° with 25 c.c. of water in a 100 c.c. flask, and to the hot liquid arc added, with continual shaking, 12 c.c. of a i : 12 potassium ferrocyanide solution and then 20 c.c. of I : 10 barium chloride solution. After vigorous shaking, the liquid is made up to volume, 0-25 c.c. of water being added to compensate for the volume occupied by the precipitate ; after standing, 25 or 50 c.c. of the clear liquid are titrated with N/io-alkali in presence of methyl orange a; i c.c. N/io- alkali = 0-0049 &'ram HaS()(I. 5. Iron (Lunge and Keler's colorimetrie method).-.....-The following are required : (a) 10% potassium thiocyanate .solution ; (b) pure ether ; (c) 8-634 grams of ferric ammonium sulphate (iron alum) and 6 c.c. of pure cone, sulphuric acid per litre ; i c.c. of this solution diluted to roo c.c. gives a solution containing o-oio gram of iron per litre ; (d) pure nitric acid, free from iron ; (0) cylinders with ground stoppers and of exactly equal height and diameter (internal and external), reading from o to 25 c.c. in tenths and having a space of at least 5 c.c. between the 25 c.c. mark and the stopper. The procedure is as follows : 25 c.c. of the solution of the sulphate pre- pared as in (i) (= i gram of substance) arc; evaporated on a steam-bath to about 5 c.c., i c.c. of nitric acid (d) being then added and the liquid heated for a few minutes, allowed to cool and diluted to 50 c.c. (solution S) ; further i c.c. of the same nitric acid is diluted to 50 c.c.. (solution N). Into one cylinder (A) are poured 5 c.c. of solution S, and into another (/>) 5 c.c. of solution N to which is added a measured volume (say, i c.c.) of the diluted ferric solution c (— o-ooooi gram Fe), the liquids in the two cylinders being made up to the same volume with water. To each cylinder are then added 5 c.c. of thiocyanate solution a and loc. c. of ether, the cylin- ders being next closed and shaken until the aqueous layers are decolorised. A comparison is then made of the intensity of colour of the ethereal layers in the two cylinders (if the coloration is light, the comparison is made alter some hours of rest). This process is repeated with different, volumes of the iron alum solution until the colours of the two ethereal layers match exactly ; o-i c.c. of the ferric solution should produce an appreciable differ- ence in colour intensity. This process is applicable to sulphates containing less than 0-25% Fe ; with larger proportions, a more dilute solution must be used. * * * Pure aluminium sulphate, A12(SO4)3 -I- i8,H2O, contains 15*33% °f A12O3 1 Chem. Zeit., 1913, pp. 805 and 814. 2 The ferrocyanide precipitates all the aluminium salt and the free sulphuric acid remains in the solution ; the barium chloride then precipitates the excess of ferro- cyanide and the free sulphuric acid, but liberates hydrochloric acid equivalent to the latter.arious sulpho-acetates are sold, mostly in solution.